1. Field of the Invention
The invention relates to papermaking wherein suction dewatering of the wet, fibrous web on the forming wire is improved by showering the web with steam as the supported web is subjected to a vacuum. More particularly the improvements related to steam showers which effect variations of web moisture content across the width of the sheet.
2. Description of the Prior Art
It is well known in the art of papermaking to employ steam showers to improve suction dewatering of pulp and paper webs at various locations on the paper machine. Such showers are especially useful at the "wet end" or forming sections of the paper machine where the webs typically exceed 50 percent moisture content by weight.
A typical shower impinges dry, saturated or superheated, steam onto the travelling web. The web, supported on a forming wire or drying belt, is simultaneously subjected to a vacuum. The vacuum pulls the steam into the sheet interior where it condenses, giving up its heat of condensation. The heat is absorbed by the water content of the web.
It is known that the moisture removal rate from the web, when subjected to a vacuum, is proportional to the square root of the ratio of surface tension to viscosity. Both surface tension and viscosity are directly proportional to temperature. Therefore, increasing moisture removal rate is a linear function of increasing sheet temperature. Hodges, "Optimum Use of Steam Shower for the Paper Machine," TAPPI Vol. 60, No. 7 (July 1977).
The increase in temperature caused by the steam shower dramatically lowers the viscosity and surface tension of the water content of the web, resulting in a more thorough extraction of water for a given vacuum volumetric rate. The use of a steam hood generally decreases the overall moisture level of the web, across its entire width.
It is the experience of papermakers, however, that significant moisture content non-uniformity across the width of the web may occur which adversely affects paper machine operation. For example on most machines the edges of the web tend to become dryer than the web's center portion. The normal variation in moisture profile may be 3-4 percent; for example, varying from 3 percent moisture at the edges to 7 percent moisture content at the center of the web.
It is further the experience of papermakers that various defects in machinery or in its operation result in "wet streaks" or areas that have relatively high moisture content with respect to surrounding web areas. Many wet streaks originate, for example, in the press section when portions of the press felts become plugged because of faulty shower systems. These streaks generally appear at unpredictable locations across the width of the web. Wet streaks can run for days before the source of difficulty is found.
The degree of uniformity of moisture content of the web, across the machine width, as the web leaves the forming section determines to a large extent the average moisture level that can be maintained in finished paper at the reel. This is so because conventional pressing and can drying are not generally designed to correct local web non-uniformity across the paper machine width.
The speed of the entire machine may be determined by the maximum wetness at the reel, even if the wet streak is only two inches wide. Compensation for one or two regions of wet streaking will necessitate a reduction in overall water content by several percent in order to build an acceptable reel. This has an important effect on machine speed and steam consumption and thus has an impact on the profitability of the operation.
The prior art describes several schemes for attempting to control the sheet moisture profile in the cross machine direction. Compartmentalized steam hoods, for example, shower a wet streak with extra steam in an effort to reduce variability and the potential for rejection at the reel for failure to meet maximum water content specifications.
Dupasquier, in U.S. Pat. No. 3,726,757 and 3,795,578 describes a steam shower divided into 11 compartments across the width of the paper machine, each equipped with a separate steam flow valve. A vacuum box, opposite the shower and under the machine wire, draws steam into the web across the entire width of the machine. Chari et al, "Profile Analysis for Evaluation of a Compartmentalized Steam Box," TAPPI Annual Meeting Preprint (Mar. 15, 1976) describes operation of the Dupasquier hood. The object of the hood is to improve the basis weight and moisture profile by individually controlling steam flow to each compartment. The Chari experiment showed that the profiling steam hood is effective in reducing longterm, cross machine moisture profile variation. The Dupasquier hood resulted in a total reduction of variance of moisture content of approximately 40 percent from a base line value without any significant change in bone dry fiber profiles.
A major disadvantage of the known compartmented steam hoods becomes evident when the steam flow to individual compartments is reduced below the fixed local volumetric rate of the vacuum system. When this occurs, the vacuum system will tend to make up flow from adjacent compartments that have higher steam flow rates. The vacuum flow might be made up from air outside the hood, but this is less likely because the geometry of the bottom discharge favors robbing adjacent compartments. This is so because compartments generally have much greater adjacent side dimensions than end walls widths under which outside air might be drawn into the hood. The tendency of a vacuum system to even up the flow of steam onto the web across the width of the web works against the moisture uniformity goal and reduces, the sharpness of control possible.
Speidel, in U.S. Pat. No. 3,945,881, describes a compartmented steam shower that attempts to cope with this problem. In Speidel, the opposing suction box drawing steam into the web, is compartmented similar to the steam hood portion. During local reduction of steam flow onto the sheet, adjacent local vacuum volumetric rate is also decreased. Thus, this apparatus reduces the possibility of one compartment robbing adjacent compartments at the cost of doubling the controlling system.
Another disadvantage of the compartmented steam hoods is that the amount of steam condensed in the web is largely dependent upon the porosity of the sheet and the capacity of vacuum available. If the steam flow is increased to a point where either of these limitations is exceeded, excess steam will blow out into the machine room. This problem becomes evident when the known compartmented hoods attempt to cope with wet streaks.
It is the object of the present invention to provide a method and apparatus for improving the uniformity of paper web moisture profiles across the width of the web.
An aim of the profiling steam hoods of this invention is to remove objectionable wet streaks in the sheet by applying greater amounts of steam over larger surface areas to those areas, thereby avoiding the porosity and vacuum capacity limitations known in the art. In this regard, increased condensation of steam in the web is achieved by increasing the contact area between the web and the steam flow. Thus, absorption can be improved without relying on the limited ability of the vacuum system to draw greater rates of steam through the web.
An object of the present invention is to provide a compartmented steam shower hood that avoids the prior art problem of the vacuum system tending to cause decreased profiling control by robbing adjacent high steam flow compartments to make up for low flow areas. Design criteria of the present invention insures that there are supplemental air flows to the fixed vacuum system when steam flows to particular compartments are reduced. Thus, a sharper controlled response to wet streaks is possible.
The profile improvement permits a higher average moisture level at the reel with reduced fear of off-specification paper. For dryer limited operations, productivity is increased. Product quality, in terms of the uniformity, is improved and energy consumption reduced.